An electric power system is unique in that aggregate production and consumption are matched instantaneously and continuously, while system elements should operate within acceptable limits. Unexpected loss of generating units or transmission lines, or errors in daily load forecast, can result in sudden imbalances between generation and consumption. Such imbalances can lead to frequency deviations from the nominal frequency of the power system. This can be problematic because generators may get disconnected by over- or under-frequency protection systems and cause even larger deviations leading to a system blackout. Loads such as rotating machines should operate at constant frequency and therefore frequency deviations can result in the interruption of various manufacturing processes.
Operational variations of renewable energy generation can also occur. These operational variations can cause system frequency variation, and can lead transmission system operators to allocate more frequency regulation reserves than in the case of dispatchable energy generation.
An ESS can be effective is addressing these issues. The ESS can function as a supplier of a frequency balancing reserve. An ESS may absorb power from the grid when the actual frequency is above a defined frequency tolerance band thereby charging the battery, and an ESS may provide power to the grid when the actual frequency is below the frequency tolerance band, in that case discharging the battery.
WO 2007/104167 describes a method of operation of an ESS in which a lower state-of-charge set-point (SoC1) and an upper state-of-charge set-point (SoC2) of the battery are determined. These set-points lie between a minimum state-of-charge (SoCmin), wherein the battery is empty, and a maximum state-of-charge (SoCmax), wherein the battery is fully charged, respectively. The BESS can be controlled such that the SoC of the battery is maintained in a preferred band between the SoC1 and SoC2.The content of WO 2007/104167 is hereby incorporated by reference herein in its entirety.
U.S. Pat. No. 5,798,633 (Larsen et al.) discloses a battery energy storage system wherein an inverter is coupled to convert direct current power from a DC source to a control frequency AC power suitable for supplementing utility power or for replacing utility power. The battery energy storage system includes a control mechanism for operating the system and either a supplemental or replacement mode in parallel with a utility power system.
WO 2005/029667 describes a system for regulating frequency of generated power. An energy storage sub-system uses one or more flywheel energy storage systems to control the system frequency. Furthermore, an open-loop control uses a difference between measured frequency and reference frequency as an input signal.
Known ESSs do not operate with optimum efficiency and have unnecessarily oversized dimensions.